Shear Effects on the Mechanical Behaviour of Internally Unstable Gap-Graded Soils

Internal instability is the soil's inability to retain fine particles that can be washed away by the action of seepage flow. Internally unstable soils threaten the safety of infrastructure, especially dams. This study carried out a discrete element method (DEM) study of internally unstable soils during shearing. The samples were compacted as densely as possible and sheared under drained conditions. At the macroscale, the evolutions of deviatoric stress and volumetric strain were studied, while at the microscale, the coordination number and the stress reduction factor were analyzed. Microscale analysis revealed the mysteries associated with internally unstable soils. The evolution of the coordination number showed that there is a shift of the fabric from overfilled to underfilled due to shear-induced deformation and volumetric increase. The stress reduction factor showed that fine particles in overfilled fabric initially carried stress, but with the increase in volume caused by shear, they loosened and became stress-free. This shows that shear-induced deformation further worsens the situation in internally unstable soils. This study's findings on shear-induced deformation worsening internal instability in soils can inform better design and maintenance of infrastructure like dams. Understanding these effects helps engineers develop improved stabilization techniques and preventative measures, enhancing the safety and durability of critical structures.